This invention relates to improved thermosetting powder paints and more particularly pertains to a curing catalyst for promoting chemical cross-linking of thermosetting polymer binders in the powder paint upon exposure to heat.
Powder paints are ordinarily manufactured from raw batch ingredients comprising resinous binders, opacifying and filler pigmentary solids, plasticizers, and other additives to provide opacity, good film properties, and adhesion to substrates. The raw batch ingredients are uniformly mixed, formed into coherent extrudate by hot extrusion, and then comminuted to form small particle powder paints which are ordinarily free flowing at normal room temperature. Powder paints usually are uniform small powders passing 325 mesh or less than about 44 microns. Powder paints contain little or no fugitive solvents and depend upon their own inherent characteristics of the powder to melt, level, coalesce, and fuse to form an attractive coherent film on the substrate. The powder must not fuse in the container and be susceptible to cold flow so as to maintain individual powder particles prior to use.
Thermosetting powder coating composition based on reactive polymers having reactive carboxyl or hydroxyl or amide groups adapted to be cross-linked by tetrakismethoxymethyl-glycoluril are shown in U.S. Pat. No. 4,118,437. Similarly, a p-toluene sulfonic acid catalyst for powder paints is shown in U.S. Defensive Publication No. 624,135. However, melamine powder coatings often exhibit a gassing effect during cure and often exhibit poor heat storage stability.
It now has been found that substantially improved thermosetting powder coatings based on reactive polymers adapted to be cross-linked by aminoplast resin cross-linker can be produced by the inclusion of a minor amount of a heat curing catalyst comprising an amine salt of cyclohexyl sulfamic acid. The cured powder coatings produce pin-hole free continuous surface coatings and avoid porous or ripple surfaces which can occur due to the formation of volatile by-products formed during the fusing and curing steps.